Studies will continue on the nature of the DNA-binding proteins isolated from vaccinia virions and from intracellular replicating DNA complexes. Specifically evidence will be sought for proteins which either denature the DNA or are involved in maintenance of the compact structure. Viral transcriptase will be purified from infected cells and the ability of these other DNA-binding proteins to enhance in vitro transcription of vaccinia DNA will be determined. Extensive investigations will be made of the putative requirement for a nucleus in order to obtain full replication of vaccinia virus. Molecular studies will be conducted to determine where the block in vaccinia replication occurs in enucleated cells. Finally, a program for obtaining conditional lethal mutants of vaccinia is well underway and mutants will be collected and characterized. Continued studies on nuclear transplantations will focus on the transfer of nuclei from one type of differentiated cell into the cytoplasm of another type of differentiated cell. Specific attention will be placed on the inducibility of differentiated functions by cytoplasmic influences. Of particular interest is transplantations involving Mouse L cell nuclei and Rat hepatoma cytoplasts. It has been observed that such transplantations result in the induction by glucocorticoid hormones of a mouse tyrosine aminotransferase. Additional studies on karyoplast regeneration will also be conducted.